The development of the cloaca in the human embryo

Abstract

Subdivision of cloaca into urogenital and anorectal passages has remained controversial because of disagreements about the identity and role of the septum developing between both passages. This study aimed to clarify the development of the cloaca using a quantitative 3D morphological approach in human embryos of 4–10 post‐fertilisation weeks. Embryos were visualised with Amira 3D‐reconstruction and Cinema 4D‐remodelling software. Distances between landmarks were computed with Amira3D software. Our main finding was a pronounced difference in growth between rapidly expanding central and ventral parts, and slowly or non‐growing cranial and dorsal parts. The entrance of the Wolffian duct into the cloaca proved a stable landmark that remained linked to the position of vertebra S3. Suppressed growth in the cranial cloaca resulted in an apparent craniodorsal migration of the entrance of the Wolffian duct, while suppressed growth in the dorsal cloaca changed the entrance of the hindgut from cranial to dorsal on the cloaca. Transformation of this ‘end‐to‐end’ into an ‘end‐to‐side’ junction produced temporary ‘lateral (Rathke's) folds’. The persistent difference in dorsoventral growth straightened the embryonic caudal body axis and concomitantly extended the frontally oriented ‘urorectal (Tourneux's) septum’ caudally between the ventral urogenital and dorsal anorectal parts of the cloaca. The dorsoventral growth difference also divided the cloacal membrane into a well‐developed ventral urethral plate and a thin dorsal cloacal membrane proper, which ruptured at 6.5 weeks. The expansion of the pericloacal mesenchyme followed the dorsoventral growth difference and produced the genital tubercle. Dysregulation of dorsal cloacal development is probably an important cause of anorectal malformations: too little regressive development may result in anorectal agenesis, and too much regression in stenosis or atresia of the remaining part of the dorsal cloaca.

Extrinsic innervation of the pelvic organs in the lesser pelvis of human embryos

“…As the caudal body axis unfolded (Kruepunga et al . 2018), the angle between both planes straightened to 160–170° at CS22. Concomitantly with the unfolding of the embryonic axis, the line through the subpubic arch and vertebra S5 unfolded from CS16 onwards (panels c–f).…”

“…The caudal unfolding involved the lumbar and sacral regions until CS17 and was confined to the sacral region between CS17 and 10 weeks of development (Kruepunga et al . 2018). A striking feature of the umbilical arteries was the absence of extrinsic enteric nerves on their ventral side (not shown).…”

“…The marked curvature of the umbilical arteries in the sagittal plane at CS22, with the most caudal point lateral to the bladder trigone [the most cranial region of the urogenital part of the cloaca (Kruepunga et al . 2018)] showed that the straightening of the body axis in the late embryonic period represented axial growth of the dorsal part of the body, which was matched ventrally by the development of the ventral body wall from the lower thoracic dermomyotomes (Mekonen et al . 2015) and the fundus of the bladder from the allantois (van der Putte, 2005; Kruepunga et al .…”

“…2015) and the fundus of the bladder from the allantois (van der Putte, 2005; Kruepunga et al . 2018). We observed the configuration of the umbilical arteries, as shown in panel E in another CS22 embryo, while the configuration as shown in panel F was seen all four CS23 embryos studied, showing that the formation of the infra‐umbilical ventral body wall proceeds rapidly.…”

Journal of Anatomy January 2020

“…The Journal of Anatomy Best Paper Prize 2018 was awarded to -'The development of the cloaca in the human embryo'. (Kruepunga et al, 2018) Journal of Anatomy Runner-Up Best Paper Prize 2018 was awarded to -'Intraganglionic macrophages: a new population of cells in the enteric ganglia'. (D ora et al, 2018) This year we published 161 such papers covering the breadth of the Anatomical Sciences.…”